Dispersion strengthening of lead



April 25, 1967 D, H. ROBERTS DISPERSION STRENGTHENING OF LEAD FiledMarch 29, 1963 BALL MlLLED SURFACE 0mm 1 4 a OXIDE 7,(WE1GHT) 0 0 0 0 0O 0 0 0 0O 7 PD United States Patent 3,315,342 DISPERSION STRENGTHENING0F LEAD David Henry Roberts, Harlow, England, assignor, by mesneassignments, to St. Joseph Lead Company, New York, N.Y., a corporationof New York Filed Mar. 29, 1963, Ser. No. 269,030 Claims priority,application Great Britain, May 21, 1962, 19,517/ 62 4 Claims. (Cl.29-4205) This invention is concerned with the production of hardenedlead for industrial purposes.

The special properties of lead make it valuable in the electricalindustry for cable sheathing and the construction of batteries. It isalso useful for radiation shielding. Further, its resistance tocorrosion makes it valuable in the chemical industry. Its disadvantage,however, is its very low tensile strength, which means that a structuremade of ordinary lead has to be supported by parts made of another metale.g. copper or steel, otherwise the structure would suffer seriousdistortion from the eflFects of its own weight in the course of time. Itis therefore desirable to be able to produce lead in a strengthenedform.

A method has already been proposed for forming sheets ofdispersion-strengthened lead from lead powder by extruding the leadpowder through a die and then rolling the resultant bar along and acrossthe extrusion direction. This procedure breaks up the oxide coatings onthe lead particles, distributes the resultant fragments of oxidethroughout the lead, and causes the lead particles to become weldedtogether.

The expression oxide coatings in the foregoing means coatings formed onthe surfaces of the lead particles as a result of the exposure of thepowder to the action of the atmosphere. Throughout the remainder of thisspecification the expression is to be taken as meaning coatings eitherformed in the same way on the lead particles by the action of theatmosphere on them or else formed on particles by the actions,simultaneously or in succession, both of the atmosphere and one or moreother agents which tend to cause oxidation of the lead. Such coatingsconsist substantially of one or more of the oxides of lead and arecommonly called oxide coatings but other materials, such as thecarbonates and hydroxides of lead may also be present in them.Similarly, oxide particles through the remainder of this specificationis to be taken as meaning particles of the oxide coatings as describedabove.

We have discovered that dispersion-strengthened lead may be obtainedfrom partly oxidised lead powder by simply extruding it, provided thatthe extrusion ratio is such that the process of extrusion breaks up theoxide coatings on the lead particles, distributes the resultantfragments of oxide throughout the lead and causes the lead particles tobecome welded together. The degree of deformation to which the leadparticles need to be subjected during extrusion in order to produce adispersion strengthened extrusion product depends mainly upon the meanthickness of the oxide films on the particles. The proportion ofdispersed oxide in the extrusion product, however, and hence thestrength of this product will depend both upon the means thickness ofthe oxide films on the particles and upon their mean initial size. Thelarger the mean particle size for a given film thickness, the smallerthe proportion of dispersed oxide in the extrusion product andconversely. In a typical procedure for the fabrication of a bar ofdispersion strengthened lead, lead particles which will pass through a300 B8. Sieve and which have a mean oxide film thickness of 03 areextruded at an extrusion ratio of 20:1. No subsequent rolling of theextrusion product will be necessary in order to complete thedispersion-strengthening process.

We have also discovered that the strength of a dispersion-strengthenedlead article formed by simply extruding lead powder can be still furtherincreased by subjecting the lead powder to be extruded to a processwhereby the lead oxide content of the powder is increased without anysignificant increase in the thickness of the oxide coatings on thesurfaces of the individual lead particles.

The present invention provides a method for producingdispersion-strengthened lead from lead powder, which comprisessubjecting the lead powder to a mechanical process whereby the oxidecoatings on the lead particles are broken up and the resultant oxidefragments forced into the lead particles themselves, while furtheroxidation takes place on fresh surfaces, and thereafter extruding thelead powder to produce solid lead containing a dispersion of oxide.

Typically the mechanical process comprises milling the lead powder in avibration ball mill and this process should be carried out under suchconditions that the lead exposed as a result of the break-up of theoxide coatings will readily re-oxidise.

The purpose of the initial mechanical treatment is to increase the oxidecontent of the lead powder in such a way that the proportion ofdispersed oxide in the extruded lead is increased. This results in anincrease in the strength of the dispersion-strengthened lead.Experiments have shown that there is a substantially linear relationshipbetween the ultimate tensile strength (U.T.S.) of the extruded lead andits content of dispersed oxide, so that to achieve the highest strengthshigh oxide contents are necessary. Further, it is found that, for agiven oxide content, dispersion strengthened lead formed by extrudinglead powder which has been subjected to a mechanical process of the typedescribed above, is stronger and more ductile than dispersionstrengthened lead formed by simply extruding the surface oxidisedpowder. This is illustrated in the graph shown in the accompanyingdrawing.

Atempts to increase substantially the dispersed oxide content of theextruded lead, and thereby to increase its strength, by simplyincreasing the thickness of the oxide coating on the lead particles byheating the powder to be extruded, have not proved successful. This isbecause the thickened coatings are too tough to be broken up properly bythe extrusion process with the result that unbroken networks of oxideare formed in the extrusion product. Consequently, oxide dispersion andparticle bonding in the extrusion product are poor.

Preliminary ball-milling of the particles avoids this difficulty bybreaking up the oxide coatings initially present on the particles, andpunching the oxide fragments into them. The milling also exposes cleanlead on which, provided oxygen is present, a new thin layer of oxide isformed and then in turn broken up and punched into the lead and so on.In the presence of oxygen the process is thus a continuous one. Theevidence for this is that when lead powder is treated in a closedair-tight ball mill, large agglomerates are formed due to the coldwelding of the clean oxide-free surfaces of the lead. Further,experiments on ball milling lead powder in an inert gas have shown thatno oxidation occurs and material extruded from this powder has only avery slight increase in strength. This slight increase probably resultsfrom a better dispersion in the extrusion product of the oxide initiallypresent on the particles due to the fragmentation of the oxide coatingsin the ball mill.

Experience with the ball-milling process has shown that (a) the higherthe proportion of the balls in the charge the faster the oxidation,

(b) the higher the temperature, the faster and more extensive theoxidation, and

(c) if continuous oxidation is possible then there is no agglomerationof the powder particles due to cold welding.

For the initial ball-milling step a vibration ball mill is preferredsince it gives more severe working of the lead powder than a simpletumble ball mill. As shown above, the particles of lead powder treatedin this way will contain oxide particles within them, in addition to asurface coating (due in part to oxidation during ball-milling) and it isfound that the oxide dispersion in material extruded from this powder isvery good. It has so far only proved possible to extrude lead powdercontaining up to 7% by weight of oxide and thereby to produce aconsolidated extrusion product, when the extrusion is carried out atroom temperature. It is possible however successfully to extrude powderscontaining larger proportions of oxide by raising the extrusiontemperature.

The practice of the invention will be better understood from thefollowing description of the typical conditions for the production of abar of dispersion strengthened lead.

(a) 1 volume of commercial air atomised lead powder which will pass a300 B.S. Sieve (particle sizes 53 down to dust) is ball-milled at about70 C. for 6 hours in a vibration ball mill containing 1 volume of /2diameter steel balls. By this is meant that, for example, a beakerful ofpowder and a beakerful of balls are placed in the mill. In the case ofthe balls, therefore, it is the gross volume rather than the actualvolume of balls themselves we are concerned with, in view of the largespaces between them. The design of the ball-mill must be such that theoxygen used up during the ball-milling is continually replaced.

(b) The ball-milled powder is extruded at 25 C. through a 0.1" diameterextrusion die, the extrusion ratio being 40:1. '(This is the ratio ofthe cross-sectional area of the chamber to that of the extrusionproduct.)

Some typical results are given in Tables 1 and 2.

Conditions for milling Milling time, hours Wt. percent Oxide Charge 1volume Lead powder to 1... Volume of diam Steel balls Total charge 300gms Atmosphere: Oxygen TABLE 2.STRESS-RUPTURE PROPERTIES OF LEAD ANDDISPERSION-STRENGTHENED LEAD 1 No failures after 1,000 hours.

The present invention is not confined to the production ofdispersion-strengthened lead from lead powder. It may equally beemployed for the production of dispersion-strengthened lead alloy from apowdered alloy. It is found in general however, that particles of a leadalloy oxidise less readily than lead particles so that in some caseslead alloy particles need to be subjected to mechanical treatment for alonger time than lead particles, or under conditions which increase therate of oxidation of the alloy particles, or both. The rate of oxidationof the particles may be increased by ball milling them in oxygen at atemperature above room temperature. A typical alloy which may behardened by the methods of this invention is the so-called cablesheathing alloy B containing 98.2% lead and 0.8% antimony.

What I claim is:

1. A method for producing dispersion-strengthened lead from lead powder,which comprises subjecting the lead powder to a mechanical processwhereby the oxide coatings on the lead particles are broken up and theresultant oxide fragments forced into the lead particles themselves,while further oxidation takes place on fresh surfaces, and thereafterextruding the lead powder to produce solid lead containing a dispersionof oxide.

2. A method according to claim 1, in which the lead powder is subjectedto milling in a vibration ball mill.

3. A method according to claim 2, in which arrangements are made forcontinual changing of the air or oxygen in the ball mill to promoteoxidation of the lead powder.

4. A method according to claim .1, in which a dispersion-strengthenedlead alloy is produced from a lead alloy powder.

References Cited by the Examiner UNITED STATES PATENTS 3,066,391 12/1962 Vordahl. 3,098,293 7/1963 Ebdon 29-528 3,189,989 6/1965 Ebdon29-420.5

JOHN F. CAMPBELL, Primary Examiner.

P. M. COHEN, Assistant Examiner.

1. A METHOD FOR PRODUCING DISPERSION-STRENGTHENED LEAD FROM LEAD POWDER,WHICH COMPRISES SUBJECTING THE LEAD POWDER TO A MECHANICAL PROCESSWHEREBY THE OXIDE COATINGS ON THE LEAD PARTICLES ARE BROKEN UP AND THERESULTANT OXIDE FRAGMENTS FORCED INTO THE LEAD PARTICLES THEMSELVES,WHILE FURTHER OXIDATION TAKES PLACE ON FRESH SURFACES, AND THEREAFTEREXTRUDING THE LEAD POWDER TO PRODUCE SOLID LEAD CONTAINING A DISPERSIONOF OXIDE.